Hypotension following traumatic brain injury (TBI) is recognized as an important secondary insult that is associated with adverse outcome. We aimed to describe the relationship between actual levels of admission blood pressure and Glasgow Outcome Scale (GOS) at 6 months. Individual patient data from the IMPACT database were available on systolic (N = 6801) and mean arterial (N = 6647) blood pressure. Regression models with restricted cubic spline functions were used to explore the shape of the relationships between blood pressure and outcome in unadjusted and adjusted analyses. Proportional odds methodology was applied to quantify the strength of the associations across the full range of the GOS. Analyses were performed to search for threshold values. A smooth U-shaped relationship was observed between systolic (SBP) and mean arterial (MABP) blood pressures and outcome, without any evidence of an abrupt threshold effect. Best outcomes were observed for values of SBP of the order of 135 mm Hg and for values of MABP of the order of 90 mm Hg. Both lower (OR 1.53; 95% CI: 1.31-1.80) and higher levels (OR 1.42; CI: 1.20-1.68) of SBP and lower (OR 1.30; CI 1.12-1.51) and higher levels of MABP (OR 1.45; CI 1.19-1.76) were associated with poorer outcome. These findings were consistent across studies. The relationship between high blood pressure level and poorer outcome largely disappeared on adjusted analysis. Current guidelines for the management of blood pressure in TBI focus on the avoidance of hypotension as defined by SBP < 90 mm Hg. Our finding of a smooth relationship with improving outcome as SBP increases up to 135 mm Hg, while not supporting a strong causal inference, does suggest that current guidelines need to be reconsidered.